AT Newtown High School, Adam Lanza had trouble relating to fellow students and teachers, but that was only part of his problem. He seemed not to feel physical or emotional pain in the same way as classmates.

Richard Novia was the school district's head of security until 2008 and advised the school technology club that Adam and his older brother belonged to. He said Lanza clearly "had some disabilities."

"If that boy would've burned himself, he would not have known it or felt it physically," Novia told The Associated Press in a phone interview. "It was my job to pay close attention to that."

Novia was responsible for monitoring students as they used soldering tools and other potentially dangerous electrical equipment. He recalled meeting with school guidance counsellors, administrators and with the boy's mother, Nancy Lanza, to understand his problems and find ways to ensure his safety.

"He would have an episode, and she'd have to return or come to the high school and deal with it," Novia said, describing how the young man would sometimes withdraw completely "from whatever he was supposed to be doing," whether it was sitting in class or reading a book.

An undated photo shows Adam Lanza posing for a group photo of the technology club at Newtown High School. Picture: AP

Adam Lanza "could take flight, which I think was the big issue, and it wasn't a rebellious or defiant thing," Novia said. "It was withdrawal."

As authorities continue a wide-ranging investigating into the second-deadliest school shooting in US history, many are trying to understand what led the young man to kill his mother in their home and then slaughter 26 children and adults at a Connecticut school before taking his own life. The children were ages 6 and 7.

An official says Lanza had Asperger's syndrome, but experts say there's no connection between the disorder and violent behaviour.

Asperger's is a mild form of autism often characterized by social awkwardness.

The official spoke on condition of anonymity because the person was not authorized to discuss the unfolding investigation.

A 2005 picture of Adam Lanza, who carried out one of the deadliest school shootings in the US.

American psychologist Eric Butter says research suggests people with autism have a higher rate of aggressive behaviour than the general population. He says this includes outbursts, shoving or angry shouting, but typically does not include planned, intentional violence.

Adam and his older brother, Ryan, in their teens had been members of the tech club, which offered students a chance to work on computers, videotape school events and produce public-access broadcasts.

It was popular among socially awkward students. But Adam, while clearly smart, had problems that went beyond an adolescent lack of social skills, Novia said.

"You had yourself a very scared young boy, who was very nervous around people he could trust or he refused to speak with," Novia said.

The club provided a setting for students to build lasting friendships. But while other members were acquainted with Adam, none was close to him.

World reacts in horror after a shooting rampage in the US that killed 28 people, including 20 children. Sunita Rappai reports.

"Have you found his best friend? Have you found a friend?" Novia asked. "You're not going to. He was a loner."

Adam was not physically bullied, although he may have been teased, Novia said.

The club gave the boy a place where he could indulge his interest in computers. His anxieties appeared to ease somewhat, but they never disappeared. When people approached him in the hallways, he would press himself against the wall or walk in a different direction, clutching tight to his black briefcase.

"The behaviour would be more like an 8-year-old who refuses to give up his teddy bear," Novia said. "What you knew with Adam is it was a possession. It was not a possession to be put at risk."

Even so, Novia said, his primary concern was that Adam might become a target for abuse by his fellow students, not that he might become a threat.

Briefing reporters in Newtown, Connecticut State Police Lieutenant Paul Vance said police have uncovered "very good" evidence that should help determine why a gunman forced his way into an elementary school and killed 20 children in one of the worst shooting rampages in U.S. history. Rough Cut (no reporter narration)

"Somewhere along in the last four years, there were significant changes that led to what has happened," Novia said. "I could never have foreseen him doing that."

Jim McDade, who lives a few houses from where Nancy Lanza was killed, said his family became acquainted with the two brothers and their mother because their children were about the same ages and rode the school bus together.

"There was certainly no indication of anything unusual that lets you think that a kid's going to do something like that," said McDade, who works in finance in New York City. "There was nothing that would indicate anything going on behind the scenes that would lead to this horrible mess."

Police say the gunman who slaughtered 26 people at a US school forced his way into the building.

"He was very different and very shy and didn't make an effort to interact with anybody," she said.

DeVivo said Lanza always carried a briefcase and wore his shirts buttoned up to the top button. She said he seemed bright but never really participated in class.

"Now looking back, it's kind of like 'OK, he had all these signs,' but you can't say every shy person would do something like this."

A police car was parked in the driveway of the Stamford, Connecticut, home of Lanza's father, Peter Lanza to stop reporters who tried to approach the house.

Mr Lanza has released a statement on the tragedy though WFSB television in Connecticut.

"Our hearts go out to the family and friends who lost loved ones and to all those who were injured. Our family is grieving along with all those who have been affected by this enormous tragedy," the statement said.

"No words can truly express how heartbroken we are. We are in a state of disbelief in trying to find whatever answers we can," it continued.

A sheriff in New Hampshire, where Nancy Lanza once lived, read a statement from her family, saying they are trying to understand "the tremendous loss."

Rockingham County Sheriff Michael Downing said Lanza's brother James Champion, a retired police captain in New Hampshire, and other relatives express their "heartfelt sorrow" and "the whole family is traumatized by this event."

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